6 eV. Oxygen molecules can be dissociatively absorbed on the oxygen vacancies induced by doping N, thereby leading to a slight shift to lower binding energy of O 1 s of TiO2 lattice oxygen (Ti-O-Ti) [18]. Figure 3 High-resolution XPS spectra. Of the (a) Ti

2p, (b) O1s, (c) N 1 s, and (d) V 2p for N-TiO2, VN0, and VN3 samples. Figure  3c shows the high-resolution XPS spectra and corresponding fitted XPS for the N 1 s region of N-TiO2, VN0, and VN3. A broad peak extending from 397 to 403 eV is observed for all samples. The center of the N 1 s peak locates at ca. 399.7, 399.6, and 399.4 eV for N-TiO2, VN0, and VN3 samples, respectively. These three peaks are higher than that of typical binding energy of N 1 s (396.9 eV) in TiN [19], indicating that the N atoms in all samples interact strongly with O atoms [20]. The binding energies of 399.7, 399.6, and 399.4 eV here are attributed to the CT99021 chemical structure oxidized nitrogen similar to NO x species, PD0332991 which means Ti-N-O linkage possibly formed on the surface of N-TiO2, VN0, and VN3 samples [21–23]. The concentrations of V and N in VN3 derived from XPS analysis were 3.38% and 4.21% (at.%), respectively. The molar ratios of N/Ti on the surface of N-TiO2 and VN3 were 2.89% and 14.04%, respectively, indicating obvious increase of N doping content by hydrothermal treatment

with ammonium metavanadate. As shown in Figure  3d, the peaks appearing at 516.3, 516.9, 523.8, and 524.4 eV could be attributed to 2p3/2 of V4+, 2p3/2 of V5+, 2p1/2 LDN-193189 research buy of V4+, and 2p1/2 of V5+[24, 25]. It was established that the V4+ and V5+ions were successfully incorporated into the crystal lattice of anatase TiO2 and substituted for Ti4+ ions. UV-vis DRS spectra analysis UV-vis diffuse reflectance spectra of N-TiO2 and V, N co-doped TiO2 nanotube arrays are displayed in Figure  4.

The spectrum obtained from the N-TiO2 shows that N-TiO2 primarily absorbs the ultraviolet light with a wavelength below 400 nm. For the V, N co-doped TNAs samples of VN0.5 and VN1, the UV-vis diffuse reflectance spectroscopy (DRS) spectra present a small red shift of adsorption edge and a higher visible light absorbance. With the increase of co-doping amount, an obvious red shift of the absorption edge and enhanced visible light absorbance were observed 4��8C for the VN3 and VN5 samples. However, no obvious change of visible light absorbance was found for VN0, which indicates that the visible light absorbance of co-doped samples may be due to the contribution of both interstitially doped N and substitutionally doped V. Kubelka-Munk function was used to estimate the band gap energy of all samples by plotting (α ℎv)1/2 vs. energy of absorbed light. The calculated results as shown in Figure  4b indicated that the band gap energies for N-TiO2, VN0, VN0.5, VN1, VN3, and VN5 are 3.15, 3.15, 2.96, 2.92, 2.42, and 2.26 eV, respectively.